The present invention relates to an apparatus for manufacturing a light guiding plate used in a backlight device of a liquid crystal display device and its method.
The so-called edge light type light guiding plate, which lets in light from one side surface (light incident surface) of a transparent plate that is a square plate-like member, and which outputs the incident light from a surface (light output surface), is used in a backlight device of a liquid crystal display device for a word processor, personal computer, thin type television and the like. Such a backlight device is provided with a tubular light source placed on at least one side surface of the light guiding plate, and an element (hereinafter referred to as a xe2x80x9cdeflection elementxe2x80x9d) placed on a surface (light reflective surface) opposite to the light output surface that outputs light so as to change an angle of light that passes through the light guiding plate or an angle of light that is reflected by the light guiding plate.
Incident light from the light incident surface of the light guiding plate is changed in its direction at the light reflective surface and light output surface and the like, and outputs light from the light output surface. Moreover, light is totally reflected by the light reflective surface and transmits in the light guiding plate. Generally, the density distribution of deflection element and the shape thereof are determined such that luminance of light output from the light output surface becomes uniform at the entire surface of the light output surface.
The aforementioned deflection element can include the following three types {circle around (1)} to {circle around (3)}.
{circle around (1)} The surface of light guiding plate is coated with white ink that scatters or reflects light;
{circle around (2)} Dot pattern by which light is scattered or reflected is formed on the surface of light guiding plate; and
{circle around (3)} A light diffusion particle is dispersed in the light guiding plate.
The light guiding plate of type {circle around (1)} is manufactured by screen printing and the like. However, when the film thickness of print layer with white ink becomes uneven, distribution occurs in reflective power, so that luminance of light output from the light output surface becomes uneven. Further, if dust in the air is mixed into white ink during a print operation process and adhered onto the print side, light scattering is caused by dust, so that evenness of luminance cannot be obtained. Furthermore, the light guiding plate of type {circle around (3)} has difficulty in dispersing the light diffusion particle in a base material with high reproducibility in order to attain a predetermined density distribution.
For the above reason, type {circle around (2)}, that is, the light guiding plate provided with the dot pattern (hereinafter referred to as xe2x80x9cdotsxe2x80x9d), that is, the deflection element, which scatters and/or reflects light onto the surface, is used as a backlight device of a liquid crystal display device in many cases.
Conventionally, as a method for manufacturing the light guiding plate having dots, the manufacturing method with an injection moulding as disclosed in U.S. Pat. No. 5,776,636 is generally used. However, in the manufacturing method with the injection moulding, there is a problem existing in that the large-size injection moulding machine is needed in accordance with an enlargement of the light guiding plate resulting in rising in the manufacturing cost. Another problem exists in that a relatively long modeling time for pressuring and cooling was required to ensure high transcription rate.
As a method for solving the above problems, attention is focused on a method in which a resin substrate, made of acrylic, polycarbonate, polystyrene, and the like molded in the form of sheet beforehand, is subjected to surface treatment so as to manufacture a light guiding plate. As one example, Japanese Patent Laid-Open No. 11-147255 discloses a method in which the dots are formed onto the substrate of resin surface by a cylindrical roller having a pattern, corresponding to the dots, engraved on the surface so as to manufacture a light guiding plate.
In the manufacturing method described in Japanese Patent Laid-Open No. 11-147255, the dot pattern is engraved on the surface of the cylindrical roller. However, it is extremely difficult to directly engrave dots, having depths of about several xcexcm to 50 xcexcm, on the cylindrical roller. Further, in order to engrave the dots on the surface of cylindrical roller, the cylindrical roller must be manufactured for each light guiding plate having a different dot pattern, and this increases time required for working the cylindrical roller and rises in the manufacturing cost.
Moreover, in the manufacturing method described in Japanese Patent Laid-Open No. 11-147255, a plurality of mold rollers must be prepared in order to form high density patterns such that adjacent patterns such as three continuous V-trenches are arranged close to each other in a desired form. This increases time required for working these cylindrical rollers and rises in the manufacturing cost.
Furthermore, in the manufacturing method described in Japanese Patent Laid-Open No. 11-147255, the resin substrate conveyed by a conveyer belt is sandwiched between two cylindrical rollers (a mold roller and a support roller) so as to form the dots. However, in some cases, the resin substrate was shifted from a predetermined position (predetermined position to the cylindrical roller) on the conveyer belt by vibration and the like, so that the dot pattern was not formed onto the predetermined position of the resin substrate. In order to solve such a problem, it was needed that the resin substrate and cylindrical roller be positioned correctly.
Generally, the resin substrate has a thickness deviation of more than 100 xcexcm. In the manufacturing method described in Japanese Patent Laid-Open No. 11-147255, since the resin substrate is passed between the mold roller and the support roller with a predetermined distance, making it difficult to form the dot pattern having depths of about several xcexcm to 50 xcexcm without any defect.
An object of the present invention is to provide a light guiding plate manufacturing apparatus and its method that can solve the problems of the above-mentioned prior art and manufacture a light guiding plate, particularly a large-size light guiding plate with accuracy and at low cost.
In order to solve the aforementioned problems, the light guiding plate manufacturing apparatus of the present invention comprises a cylindrical roller, which has a heater built-in, rotating while fixing a stamper having a dot pattern formed on its surface; substrate fixing means for reciprocating along a lower portion of the cylindrical roller in accordance with rotation of the cylindrical roller as fixing a resin substrate; and pressing means for pressing the stamper, which is fixed to the cylindrical roller, onto a surface of the resin substrate, which is fixed to the substrate fixing means, with a constant pressure.
Here, it is preferable that the cylindrical roller be made of the material whose thermal expansion coefficient is higher than that of the stamper fixed to the cylindrical roller.
Moreover, the above-structured light guiding plate manufacturing apparatus may further comprise heating means for heating the resin substrate fixed to the substrate fixing means in advance. When a temperature of deflection under load of the resin substrate fixed to the substrate fixing means is set to T, the heating means may heat the resin substrate to temperature in the range of (Txc2x120)xc2x0 C. and the cylindrical roller may heat the stamper fixed to the cylindrical roller to temperature in the range of from no less than (T+10)xc2x0 C. to no greater than (T+30)xc2x0 C.
Moreover, the above-structured light guiding plate manufacturing apparatus may further comprise preheating means for heating a portion close to the surface of the resin substrate to temperature in the range of (Txc2x120)xc2x0 C. before the stamper is pressed by the cylindrical roller.
Still moreover, in order to solve the aforementioned problems, the light guiding plate manufacturing method of the present invention comprises the steps of fixing a stamper, having a dot pattern formed on its surface, by a rotating cylindrical roller made of material whose thermal expansion coefficient is higher than that of the stamper; fixing a resin substrate by substrate fixing means, which reciprocates along a lower portion of the cylindrical roller in accordance with rotation of the cylindrical roller; heating the resin substrate to temperature in the range of (Txc2x120)xc2x0 C. and the stamper to temperature in the range of from no less than (T+10)xc2x0 C. to no greater than (T+30)xc2x0 C. when a temperature of deflection under load the resin substrate is set to T; and pressing the stamper, which is fixed to the cylindrical roller, onto a surface of the resin substrate, which is fixed to the substrate fixing means, with a constant pressure.